1. Field of the Invention
The present invention relates to novel polyfunctional catalysts, and, more especially, to novel polyfunctional catalysts useful for the treatment of the exhaust gases and fumes emanating from internal combustion engines.
As utilized herein, by the term "polyfunctional catalysts" there are intended those catalysts which effect the oxidation of, in particular, carbon monoxide and the hydrocarbons present in internal combustion engine exhaust gases, as well as the simultaneous reduction of, in particular, the nitrogen oxides (NO.sub.x) present in such gases.
2. Description of the Prior Art
In gasoline internal combustion engines, the composition of the exhaust gases can be controlled vis-a-vis stoichiometric equilibrium in such fashion that the oxidation and the catalytic reduction of the various constituents of equilibrium afford water, carbon dioxide and nitrogen. And the means typically employed for regulating, or adjusting the composition of the exhaust gas vis-a-vis stoichiometric equilibrium are, notably, the continuous regulation of the air/fuel ratio fed into the engine, and/or the introduction of additional oxygen upstream of the catalyst. The composition of the exhaust gases thus varies, over periods on the order of one second, from a composition containing a relative excess of the oxidizing compounds (a so-called "lean" setting) to a composition containing an excess of reducing compounds (a so-called "rich" setting), and vice-versa.
In particular, the so-called "lean" setting is such that the amounts of oxygen and nitrogen oxide present are greater than those required to bring about the total oxidation of the carbon monoxide, hydrocarbons and hydrogen present. Conversely, and specifically, the so-called "rich" setting is such that the amounts of carbon monoxide, hydrocarbons and hydrogen present are greater than those required to bring about the total reduction of the oxygen and nitrogen oxides present.
Anti-pollution catalysts for treating such exhaust gases have already been proposed to this art. Thus, according to the Journal of Catalysis, 56, 321 (1979) and the 6th North-American Congress of the Catalysis Society, Chicago, March 1979, Communication H.sub.4, polyfunctional catalysts comprising cerium oxide and precious metals as the active phase are known.
And according to published Japanese Patent Application No. 3062-777, polyfunctional catalysts comprising iron oxide and previous metals as the active phase are also known.
However, such catalysts have proven to display insufficient initial activity, as well as insufficient stability over prolonged periods of time to conform to current anti-pollution requirements and standards.